Mineralogy, geochemistry and Nd isotope composition...
Transcript of Mineralogy, geochemistry and Nd isotope composition...
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Mineralogy, geochemistry and Nd isotope composition of the Rainbow hydrothermal field, Mid-Atlantic Ridge
ANA FILIPA A. MARQUES1 (�)
FERNANDO J.A.S. BARRIGA1
VALERIE CHAVAGNAC2
YVES FOUQUET3
1Address:
CREMINER / Dept. Geologia, Faculdade de Ciências, Universidade de Lisboa,
Edifício C6, Piso 4, Campo Grande, 1749-016 Lisboa, Portugal
E mail: [email protected]
Telephone: 351 21 750 00 00 (ext. 26466)
Fax: 351 21 750 00 64
2Address: National Oceanography Centre, Southampton, European Way, Southampton SO14
3ZH, United Kingdom
3Address IFREMER, Centre de Brest, Plouzané BP 70, 29280 Plouzané, France
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Abstract
Petrological, geochemical and Nd-isotopic analyses have been carried out on rock samples from the Rainbow
vent field to assess the evolution of the hydrothermal system. The Rainbow vent field is an ultramafic-hosted
hydrothermal system located on the Mid-Atlantic Ridge, characterized by vigorous high-temperature venting
(~365ºC) and unique chemical composition of fluids: high chlorinity, low pH and very high Fe and REE
contents (Douville et al. 2002). Serpentinisation has occurred under a low-temperature (<270ºC) retrograde
regime, later overprinted by a higher temperature sulphide mineralisation event. Retrograde serpentinisation
reactions alone cannot reproduce the reported heat and specific chemical features of Rainbow hydrothermal
fluids. The following units were identified within the deposit: (1) non-mineralised serpentinite, (2) mineralised
serpentinite – stockwork, (3) steatite, (4) semi-massive sulphides, and (5) massive sulphides which include Cu-
rich massive sulphides (up to 28 wt% Cu) and Zn-rich massive sulphide chimneys (up to 5 wt% Zn). Sulphide
mineralisation has produced significant changes in the sulphide-bearing rocks including enrichment in transition
metals (Cu, Zn, Fe and Co) and LREE, increase in the Co/Ni ratios comparable to those of mafic Cu-rich VMS
deposits and different 143
Nd/144
Nd isotope ratios. Vent fluid chemistry data are indicative of acidic, reducing and
high temperature conditions at the subseafloor reaction zone where fluids undergo phase separation most likely
under sub-critical conditions (boiling). An explanation for the high chlorinity is not straightforward unless
mixing with a high salinity brine or direct contribution from a magmatic Cl-rich aqueous fluid is considered.
This study adds new data, which, combined with the current knowledge of the Rainbow vent field, brings
compelling evidence for the presence, at depth, of a magmatic body, most likely gabbroic, which provides heat
and metals to the system. Co/Ni ratios proved to be a good tool to discriminate between rock units, degree of
sulphide mineralisation and positioning within the hydrothermal system. Deeper units have Co/Ni<1 and
subsurface and surface units have Co/Ni>1.
Keywords: Massive sulphides, Serpentinite, Nd isotopes, Mid-Atlantic Ridge, Rainbow vent field.
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Introduction
More than one hundred hydrothermal sites have been discovered over the past twenty-
five years along the worldwide network of mid-ocean ridges (Baker and German 2004). Most
hydrothermal sites are basalt-hosted, with seafloor venting corresponding to the visible part
of a complex and vigorous fluid circulation system active at the sub-surface. A typical
modern seafloor sulphide deposit encompasses a consolidated sulphide mound, underlain by
a sub-seafloor stockwork and covered by active and inactive chimney structures,
hydrothermal crusts, metalliferous sediments and sulphide debris (Goodfellow and Franklin
1993; Fouquet et al. 1993a, 1993b; Rona et al. 1993; Alt 1995; Hannington et al. 1995;
Herzig and Hannington 1995; Humphries et al. 1995; Fouquet et al. 1996). These features,
commonly found in modern seafloor deposits, correlate well with those of ancient volcanic-
hosted massive sulphide (VMS) deposits (Doyle 2003). Both ancient ore-grade VMS deposits
and their modern analogs result from sub-seafloor heat driven seawater circulation, reacting
with crustal or upper mantle rocks, and producing syngenetic accumulations of massive
sulphides hosted by submarine volcanic successions (Large 1992 and references therein;
Barrie and Hannington 1997).
There is a strong relation between the base metal contents in VMS deposits and the
nature of their host rocks (Barrie and Hannington 1997). Accordingly, VMS deposits (ancient
and modern) can be divided into five categories: (1) mafic, (2) bimodal-mafic, (3) mafic-
siliciclastic, (4) bimodal-felsic and (5) bimodal-siliciclastic. Mafic VMS are smaller, Cu-rich
and Pb-poor deposits and are generally associated with synvolcanic intrusions considered to
be the heat and metal sources (Barrie and Hannington 1997). Several modern seafloor
sulphide deposits along the Mid-Atlantic Ridge (MAR) (e.g. TAG, MESO, Snake Pit) have
characteristics comparable to those of mafic VMS deposits in which sediment-starved, high
temperature deposits are Cu and Co rich with high Cu/Zn ratios, contrasting with lower
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temperature sedimented ridges that are Zn and Pb-rich (Kase et al. 1990; German et al. 1993;
Fouquet et al. 1993; Goodfellow and Franklin 1993; Rona et al. 1993; Tivey et al. 1995;
Langmuir et al. 1997; Herzig et al. 1998; Münch et al. 1999; Charlou et al. 2000; Lawrie and
Miller 2000; Douville et al. 2002). Although in mafic rocks Ni is in excess relative to Co, the
related VMS hold Co-rich sulphides, most likely due to the more chalcophile behaviour of Co
(Hawley and Nichol 1961).
The recent discovery of ultramafic-hosted seafloor hydrothermal systems at the MAR
adds a new perspective to the dynamics of seafloor hydrothermal processes and the influence
of host rocks. So far, there are five known ultramafic-hosted hydrothermal sites at the MAR:
Rainbow, Logatchev, Lost City, Saldanha and Menez Hom, all linked to serpentinisation
reactions and high methane anomalies (Batuyev et al. 1994; German et al. 1996; Donval et al.
1997; Fouquet et al. 1998; Fouquet et al. 2001; Kelley 2001; Barriga et al. 2002). Rainbow
and Logatchev are vigorous high-temperature systems with high bulk Cu/Zn ratios, high Cu
and Co and low Pb, producing metal-rich black smoker fluids while Saldanha and Menez
Hom are low temperature diffuse systems (<10ºC) (Batuyev et al. 1994; Bogdanov et al.
1995; Lein et al. 2001; Fouquet et al. 2001; Douville et al. 2002; Barriga et al. 2002; Marques
2005 PhD thesis in preparation). The Lost City field contains carbonate-brucite chimneys
venting high pH fluids (pH 9-10) at intermediate temperatures (40-75ºC) (Kelley et al. 2001;
Früh-Green et al. 2003; Allen and Seyfried 2004). Serpentinisation driven hydrothermalism
seems to be the leading process occurring at Lost City, Saldanha and Menez Hom (Barriga et
al. 1998, 2004; Kelley et al. 2001; Früh-Green et al. 2003; Allen and Seyfried 2004, Costa
2005).
The Rainbow hydrothermal vent field is a vigorous hydrothermal system hosted in
completely serpentinised peridotites. Rainbow vent fluids are characterized by the highest
temperature reported for MAR fluids (365ºC), the highest chloride concentration (750 mM),
the lowest end-member pH (2.8), high trace metal contents (Fe, Cu, Zn, Co, Ni) and high
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REE and K, Rb and Cs contents (Douville et al. 2002). These unique physical and chemical
characteristics have raised numerous questions concerning the processes acting at Rainbow.
Recent cruises at the Rainbow hydrothermal vent field (e.g., HEAT cruise 1994; FLORES
cruise 1997; IRIS cruise 2001) have enabled a thorough understanding of the tectonic setting,
hydrothermal fluid and particle chemistry and the record of hydrothermal activity by
sediment coring (German et al. 1996; Barriga et al. 1997; Donval et al. 1997; Douville et al.
1997; Parson et al. 1997; Gràcia et al. 2000; Cave et al. 2002; Douville et al. 2002; Edmonds
and German 2004; Chavagnac et al. 2005). However, our knowledge of the sub-surface
structure of the Rainbow active hydrothermal system remains very limited. Fluid interaction
and mineralisation processes need to be addressed in order to reach a better understanding of
the functioning of this ultramafic-hosted hydrothermal system.
In this paper, we present a detailed, comprehensive and thorough description of the
geological setting, structure and mineralogy of the Rainbow hydrothermal sulphide deposit
and underlying stockwork, from the rock precursor serpentinite to massive sulphides. The
field data and sampling result not only from the cruises mentioned before but also, and
mainly, from the SALDANHA (1998), IRIS (2001) and SEAHMA (2002) cruises (IRIS
cruise report 2001; SALDANHA cruise report 1999; SEAHMA cruise report 2003). We have
carried out geochemical and Nd isotope analyses to characterize the Rainbow hydrothermal
deposit and to assess the evolution of its formation.
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Geological Setting
The Rainbow hydrothermal vent field is located on the MAR at the inside corner of a
non-transform offset, which links the second-order ridge segments AMAR and South AMAR
(36º14’N; 33º53’W) (Fig. 1). The vent area is composed of a group of at least 10 black
smokers expelling high-temperature (~365ºC) fluids with evidence of phase separation at
depth (Fouquet et al. 1997; Douville et al. 2002).
Fig. 1 Location of the Rainbow vent field in a non-transform offset between the AMAR and South AMAR
second-order segments at the Mid-Atlantic Ridge (MAR), south of the Azores Islands
Field observations during ROV dives, IRIS and SEAHMA cruises, coupled with
sample location data collected during the SEAHMA, IRIS and FLORES cruises have enabled
the drafting of a detailed geological map of the vent field area (Fig. 2a) (FLORES cruise
report 1998; SEAHMA cruise report 2002; IRIS cruise report 2001). In the Rainbow ridge
area, completely serpentinised peridotite outcrops, covered by whitish pelagic sediment, are
often crosscut by normal faults exposing the serpentinite. On the western limit of the active
vent area, a sub-vertical normal fault trending N-S reveals ultramafic rocks with visible
networks of sulphide-bearing veinlets (i.e. a well-defined stockwork). In the upper part of the
scarp, sulphides become more abundant, occurring as semi-massive sulphides. The active
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venting area contains numerous active and inactive sulphide chimneys that lie on top of
sulphide mounds built up mostly by the accumulation of collapsed, dead chimneys. Basalt
outcrops were found exclusively on the bottom of the South AMAR segment (W of Rainbow
ridge) and at the easternmost section of the vent field area at the summit of the Rainbow
ridge. The basalts at the ridge summit are pillow lavas and overlie the serpentinites. So far, no
contact has been recognized (Fouquet et al. 1998). Figure 2b shows a geological profile
which combines bathymetry with sample type and location (SEAHMA cruise) and ROV
observations (SEAHMA cruise).
Fig. 2 a Geological map of the Rainbow vent field area with sample location (geology and sample location from
direct observations during IRIS and SEAHMA 1 cruises and FLORES cruise report; bathymetric data and
ADELIE software from IFREMER). Line AB is the cross-section represented in (b). b Interpretative geological
cross-section, with no vertical exaggeration, representing location of samples collected during dive SH1-PL5 of
the SEAHMA1 cruise
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Sample Description
From the rock suite collected during the IRIS and SEAHMA cruises using ROV and
dredges, 128 samples were selected. Of these, 71 have been carefully studied under
transmitted and reflected light microscopy. Fig. 3 illustrates a paragenetic sequence in the
Rainbow rocks and ores and Fig. 4 contains thin section microphotographs taken from
representative samples of the ore deposit.
Fig. 3 Paragenetic sequence of minerals present in studied rocks from the Rainbow vent field. Increasing
mineral abundances are represented by increasing line thickness. Abbreviations: Srp serpentinite, Stea steatite,
SMS semi-massive sulphides, Cu-MS Cu massive sulphides, MS ch massive sulphide chimneys
Host rocks
Serpentinites (Srp) that form the substratum of the Rainbow vent field are non-
deformed and comprise serpentine-group minerals (mainly lizardite) with well-preserved
pseudomorphic textures (mesh, hourglass and bastite) dust-like euhedral grains of magnetite
(mgt I) and accessory anhedral Cr-spinel with hydrothermally altered rims (Fig. 4a, b). These
features correspond to serpentinites that resulted from simple peridotite-seawater interactions
under static, low-temperature, retrograde metamorphism. Subsequent, sigmoidal cross-
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fractures filled with fibrous γ-serpentine and carbonate-filled veins define the late and brittle
deformation stage.
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Fig. 4 Interpretative scheme through the active area of Rainbow vent field. Microphotographs taken under
reflected light, except (a) (b), (c) and (d) taken with transmitted light with crossed nicols. a Partially
serpentinised peridotite with pyroxene, olivine and serpentine mesh (after olivine). b Serpentinite with well-
developed serpentine mesh texture after olivine. c Mineralised serpentinite (stockwork) with non-
pseudomorphic featureless serpentine and coarse-grained magnetite II. d Transitional stage between stockwork
and steatite where the non-pseudomorphic serpentine is gradually replaced by talc. e Semi-massive sulphide unit
(SMS) with relics from the rock precursor (Cr-spinel and highly altered phyllosilicates). f Recrystallized Cu-rich
massive sulphide unit (Cu-MS) with intermediate solid solution/chalcopyrite exhibiting 120º jointing. g Massive
sulphide chimney (type I) with low temperature sulphide assemblage pyrite + Zn-sphalerite +
chalcopyrite/intermediate solid solution. h Massive sulphide chimney (type II) with high temperature sulphide
assemblage, pyrrhotite + Fe-sphalerite + intermediate solid solution/chalcopyrite. Abbreviations: Ccp/ISS
exsolved chalcopyrite in intermediate solid solution, Cr-spl Cr-spinel, ISS intermediate solid solution, Mgt II
magnetite II, NP-srp non-pseudomorphic serpentine, Ol olivine, Po pyrrhotite, Px pyroxene, Py pyrite, Sp
sphalerite, Srp mesh serpentine with mesh texture, Tlc talc
Sulphide-bearing Zones
Progressive percolation of high-temperature mineralising hydrothermal fluids through
the serpentinite induced the pervasive replacement of silicate mineralogical assemblages
resulting in textural changes (Fig. 4c-h). The various sulphide-bearing rocks are described
below.
Stockwork in serpentinite (St)
Pseudomorphic serpentine-group minerals, characteristic of serpentinites, are replaced
by non-pseudomorphic featureless serpentine, mainly chrysotile (Fig. 4c). The earliest
hydrothermal sulphides occur here, either disseminated or in veinlets and surrounded by the
non-pseudomorphic serpentine matrix. Euhedral to anhedral pyrite (py) grains are ubiquitous
and predate all other sulphide phases. Intermediate solid solution sulphides with exsolved
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chalcopyrite (ISS/Ccp) occur mostly as spherical aggregates or small anhedral grains
dispersed within the serpentine matrix and, occasionally, replace pyrite. Rarely, fine-grained
covellite occurs as rims surrounding ISS/Ccp. Euhedral lamellar crystals of pyrrhotite (po)
and less frequently sphalerite (sp) may occur. Dust-like magnetite (mgt I) produced during
serpentinisation disappears and a new, coarse-grained magnetite (mgt II), indicative of late
oxidizing conditions, occurs as individual and dispersed grains that often replace pyrite.
Hydrothermally altered Cr-spinel relics (Cr-spl) reveal highly reflective borders of
“ferritchromit”. Large, euhedral and radial prisms of aragonite occur along veins previously
filled by sulphides.
Steatite (Stea)
Locally, stockwork serpentinites with incipient sulphide mineralisation have
undergone a different type of hydrothermal alteration, where talc has extensively replaced
serpentine-group minerals forming steatite (soapstone) (Fig. 4d). Oxides like magnetite are
rare and relic Cr-spinel grains are highly altered. Trace sulphides may occur. Usually
pyrrhotite associated with ISS/Ccp and, more rarely, altered pyrite.
Semi-massive sulphides (SMS)
The semi-massive sulphide unit is the result of the pervasive replacement of
stockwork silicates by hydrothermal sulphides. Here, host rock relics may be preserved as
extremely altered serpentinite “islands” surrounded by sulphide masses or as altered Cr-
spinel relics that prove the ultramafic nature of the host rock (Fig. 4e). Sulphide assemblages
are similar to the ones observed in stockwork samples. Anhedral masses of pyrite are
sometimes replaced by magnetite II or by ISS/Ccp.
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Massive sulphides (MS)
Cu-massive sulphides (Cu-MS) - Cu-massive sulphides are characterized by a dense array of
ISS/Ccp sulphides with low porosity and no apparent mineralogical zonation. This unit
results from extensive replacement of the pyrite-rich SMS ore by ISS/Ccp. Some sections
show annealing (120º joints) indicative of recrystallization processes (Fig. 4f). ISS/Ccp
contains altered rims of covellite and rare fracture-filling anhydrite.
Massive sulphide chimneys (MS-ch I, II) - As opposed to Cu-MS, sulphide chimneys are
porous and frequently show concentric mineralogical zonation around a vent orifice. Textures
correspond to an interlocking network of sulphides representative of open-space type
deposition due to mixing of a hot metal-rich vent fluid with cool seawater. Rainbow
hydrothermal chimneys are sphalerite-rich (Fig. 4g, h). MS-ch type I are defined by a lower
temperature sulphide assemblage composed of euhedral pyrite overprinted by colloform
sphalerite with ISS/Ccp intergrowths (Fig. 4g). MS-ch type II contain lamellar crystals of
pyrrhotite interlocked with sulphide aggregates. The latter are zoned, with an ISS/Ccp core
rimmed by chalcopyrite, both surrounded by colloform sphalerite (Fig. 4h). Textural
evaluation of MS-ch type II indicates cogenetic sulphide formation and assemblages that are
compatible with high temperature venting.
Rodingitic rocks
A few samples are composed of relics of extremely altered rodingite-like,
serpentinised material in a matrix of aragonite, which appears to have extensively replaced
former serpentinites. Petrography reveals aggregates of interpenetrating serpentine with rare
bastite relics plus chlorite and rare garnet (grossular/hydrogrossular?) surrounded by euhedral
grains of aragonite. Large hydrothermal diopside grains that are not in textural equilibrium
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with aragonite may overprint the serpentine/chlorite aggregates. These assemblages are
indicative of Ca mobilization, following retrograde serpentinisation somewhat similar to the
rodingitization processes as described by O'Hanley (1996).
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Methodology
Electron microprobe analyses were performed using a CAMECA SX50 with 3 crystal
spectrometers at the University of Toronto. Silicates were analysed using an accelerating
voltage of 15 kV, and sample current of 15 nA with a beam diameter of 3 microns. The
sulphide and oxide routine took place under an accelerating voltage of 20 kV and sample
current of 30 nA with a beam diameter of 1 micron. A few serpentine-group minerals were
analysed at the University of Lisbon using a JEOL JCXA 733 microprobe. Whole-rock
geochemistry analyses were undertaken by ACTLABS, Canada using the LITORESEARCH
QUANT package. Procedures for rock pulverization were as follows: (1) selected samples
were diamond-sawn into small slices (< 1.5 cm thick), (2) slices were then ground using a
diamond-grinding wheel in order to prevent contamination, (3) subsequently samples were
carefully washed and dried at 25ºC, (4) dried samples were crushed using a hammer while
enclosed within thick paper envelopes to prevent metal contamination, (5) small fragments
were then powdered in an agate ring mill. For Nd isotope analyses, 100 mg to a few grams of
bulk powder samples were dissolved either in a mixture of HCl and HNO3, or HF and HNO3
in a 3:1 proportion followed by a second step, in concentrated HNO3 in a sealed savillex
beaker and left on a hotplate at 150ºC for a couple of days. The solution was then dried up
and converted to chloride form by adding 5 to 10ml of 6 M HCl. Separation of Sm and Nd
was done using a routine two column ion-exchange technique which firstly extracts REE
through a cation exchange column, followed by a separation of Nd from Sm through a second
column packed with Kel-F Teflon coated HDEHP. Nd isotope ratios were measured at the
National Oceanography Centre, Southampton, England on a seven-collector VG Sector 54
mass spectrometer. 143
Nd/144
Nd ratios were determined in multi-dynamic collection mode.
Isotope ratios were normalized to 146
Nd/144
Nd = 0.7219. Measured values for JNd-i standards
were 143
Nd/144
Nd = 0.512104 ± 10 (2sd, n = 38).
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Results
Rock sample locations and descriptions of representative samples are reported in
Table 1. Results for REE, mineral chemistry and Nd isotopic compositions are given in
Tables 2, 3 and 4, respectively.
Table 1 Sample location chart with mineralogy and rock type description. Abbreviations: ( ) relics, Anh
anhydrite, Arg aragonite, Chl chlorite, Di diopside, ISS/Ccp exsolved chalcopyrite in intermediate solid solution,
Mgt magnetite, Ol olivine, Po pyrrhotite, Px pyroxene, Py pyrite, Sp sphalerite, Spl Cr-spinel, Srp serpentine-
group minerals and Tlc talc
Dredge Start Dredge Finish Rock Type Sample
Lat Lon Lat Lon
Mineralogy
IR-DR-01-A-05 36º19.2 N 33º54 W 36º16.2 N 33º45 W (Ol), (Px), (Spl), Srp, Chl, Mgt Serpentinite
IR-DR-02-B-04 36º16.2 N 33º54 W 36º13.8N 33º45 W (Spl), Srp, Chl, Mgt Serpentinite
IR-DR-02-D 36º16.2 N 33º54 W 36º13.8N 33º45 W (Spl), Srp, Mgt, Py Stockwork
IR-DR-03-F 36º13.8 N 33º54 W 36º11.4 N 33º45 W (Spl), Srp, Mgt, Arg Stockwork
IR-DR-02-C-01 36º16.2 N 33º54 W 36º13.8N 33º45 W (Spl), Srp, Mgt, Py, ISS/Ccp, Po, Sp, Arg Stockwork
SH1-DR3-9-4 36º13.8 N 33º54.6 W 36º13.8 N 33º53.4 W Py, ISS/Ccp, Mgt Stockwork
SH1-DR3-9-9 36º13.8 N 33º54.6 W 36º13.8 N 33º53.4 W Py, ISS/Ccp, Mgt Stockwork
IR-DR-03-E 36º13.8 N 33º54 W 36º11.4 N 33º45 W (Spl), Tlc, Mgt, Po, ISS/Ccp Steatite
SH1-DR3-8-3 36º13.8 N 33º54.6 W 36º13.8 N 33º53.4 W (Spl), Tlc, Mgt, Po, ISS/Ccp Steatite
IR-DR-01-C-01 36º19.2 N 33º54 W 36º16.2 N 33º45 W Py, ISS/Ccp, Mgt Semi-massive sulphide
IR-DR-01-G-02 36º19.2 N 33º54 W 36º16.2 N 33º45 W (Spl), Py, ISS/Ccp, Mgt Semi-massive sulphide
SH1-DR3-2-1 36º13.8 N 33º54.6 W 36º13.8 N 33º53.4 W Py, ISS/Ccp, Mgt Semi-massive sulphide
SH1-DR3-2-3 36º13.8 N 33º54.6 W 36º13.8 N 33º53.4 W Py, ISS/Ccp, Po Semi-massive sulphide
SH1-DR3-1-2 36º13.8 N 33º54.6 W 36º13.8 N 33º53.4 W ISS/Ccp, Anh Cu-massive sulphides
IR-DR-02-L-01 36º16.2 N 33º54 W 36º13.8N 33º45 W Srp, Chl, Arg, Di, Po Rodingitic material
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Bulk-rock geochemistry
Figure 5 illustrates major and trace element variation of 15 selected samples that
represent the mineralising sequence from serpentinite to massive sulphides.
Fig. 5 Bulk-rock chemical data of selected elements (SiO2, MgO, Co, Ni, Cu, Zn, Sm/Nd, Eu/Eu*, Ce/Ce*)
from representative samples of the upper section of the hydrothermal system
SiO2 and MgO contents decrease from serpentinites towards stockwork, semi-massive
sulphides and massive sulphides, due to gradual silicate destruction with progressive sulphide
replacement. Steatites are more silica-rich due to the predominance of talc. Steatites are, at
least locally, indicative of higher silica activity and high fluid/rock ratio. Co abundances in
the unmineralised serpentinites are below 100 ppm, while Co contents in mineralised samples
increase with the amount of sulphide mineralisation, reaching high concentrations in semi-
massive sulphides (~2335 ppm Co, n=4), Cu-massive sulphides (~4770 ppm Co) and massive
sulphide chimneys (6300 ppm Co) (Marques 2005, Ph.D. thesis in preparation). Ni is absent
in Cu-rich massive sulphides and massive sulphide chimneys. Unmineralised serpentinites
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are Cu depleted (~95 ppm, n=7) and Cu abundances remain fairly low in the stockwork
(~1421 ppm, n=7) and in steatites (~257 ppm, n=3) (Marques 2005, Ph.D. thesis in
preparation). In more mineralised rocks, Cu increases sharply and becomes a major element,
in semi-massive sulphides (~11.3 wt %, n=4), Cu-rich massive sulphides (27.98 wt %; n=1)
and massive sulphide chimneys (8.35 wt %; n=1). Selenium is exclusively present in the
samples highly enriched in Cu. The high Cu-values are representative of all the strongly
mineralised samples thus, if Rainbow was on land, it would be a high-grade copper deposit.
Serpentinised peridotites are Ni-rich with lower Co (~1681 ppm Ni; ~108 ppm Co; n=7) and
Co/Ni ratios ~0.06 (Marques 2005, Ph.D. thesis in preparation) matching P-mantle values
(McDonough and Sun 1995). With the development of sulphide mineralisation, bulk-rock
Co/Ni ratios increase, resulting in stockwork semi-massive and massive sulphides with Co/Ni
>1 (Fig. 3). In absolute terms, Ni contents in serpentinite and stockwork decrease while Co
increases sharply. The peridotite precursor has high Ni and low Co, therefore it is difficult to
explain the final Co/Ni ratios and the extreme Co enrichment in the massive sulphides as
resulting solely from an ultramafic source. Zn is absent in most serpentinites and Zn
concentrations are rather low with irregular distribution except in Zn-rich chimneys.
Sphalerite is the only Zn sulphide found in the Rainbow assemblages thus its presence or
absence dictates Zn abundances (Fig. 5).
Zn-rich rocks are characteristic of massive sulphide chimneys (type I and II) and
relate to the focused venting at surface where cooler seawater interacts with hotter vent fluid.
Chimneys may contain up to 5.12 wt% Zn (e.g., Ms-ch II) (Marques 2005, Ph.D. thesis in
preparation). On the contrary, Cu-rich rocks show evidences of sub-seafloor precipitation and
recrystallization (Cu-MS) where higher temperatures can be attained. The presence, in these
amounts, of Cu, Zn, and Co is characteristic of mafic-hosted seafloor hydrothermal systems.
One may expect the contribution of a mafic metal source responsible for Cu, Zn and Co
input, along with Fe, into the system.
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Table 2 Rare earth element (REE) analysis (in ppm) of serpentinites, stockwork, steatites, rodingitic material,
semi-massive sulphides and massive sulphides from the Rainbow vent field. (n.d). not detected
Sample IR-DR-
01-A-05
IR-DR-
02-B-04
IR-DR-
02-D
IR-DR-
03-F
IR-DR-
02-C
SH1DR
3-9-4
SH1DR
3-9-9
SH1DR
3-2-3
SH1DR3
-2-1
IR-DR-
01-C-01
IR-DR-
01-G-02
SH1DR
3-1-2
SH1DR
3-8-3
IR-DR-
03-E
IR-DR-
02-L-01
BIR-1 BIR-1 DNC-1 DNC-1
Type Serp Serp St St St St St SMS SMS SMS SMS Cu-MS Stea Stea Rod measured certified measured certified
La 1.03 0.07 1.10 1.03 0.21 2.08 1.39 7.36 21.46 19.23 2.39 0.21 2.13 0.48 0.64 0.81 0.62 3.91 3.8
Ce 1.46 0.09 0.80 0.73 0.33 1.50 1.39 7.80 20.27 21.27 2.66 0.43 2.17 0.45 1.18 2.11 1.95 8.59 10.6
Pr 0.28 0.01 0.11 0.12 0.03 0.20 0.16 0.56 1.23 1.47 0.19 0.05 0.15 0.04 0.12 0.39 0.38 1.04 1.3
Nd 0.93 <0.05 0.30 0.27 0.13 0.52 0.44 1.78 3.54 4.54 0.57 0.16 0.45 0.11 0.47 2.52 2.5 4.95 4.9
Sm 0.21 0.01 0.05 0.05 0.04 0.08 0.09 0.28 0.44 0.60 0.07 0.03 0.07 0.03 0.11 1.13 1.1 1.43 1.38
Eu 0.09 0.01 0.04 0.03 0.01 0.26 0.03 0.1 0.81 0.05 0.03 0.01 0.01 0.02 0.11 0.55 0.54 0.62 0.59
Gd 0.25 <0.01 0.08 0.05 0.03 0.09 0.09 0.18 0.32 0.36 0.05 0.02 0.05 0.04 0.15 1.86 1.85 2.05 2
Tb 0.04 <0.01 0.01 <0.01 <0.01 <0.01 <0.01 0.02 0.06 0.05 <0.01 <0.01 <0.01 0.01 0.03 0.39 0.36 0.41 0.41
Dy 0.26 0.01 0.08 0.04 0.04 0.07 0.07 0.09 0.29 0.16 0.02 0.02 0.05 0.06 0.18 2.65 2.5 2.73 2.7
Ho 0.06 <0.01 0.02 <0.01 <0.01 <0.01 <0.01 0.01 0.06 0.02 <0.01 <0.01 <0.01 0.01 0.04 0.55 0.57 0.59 0.62
Er 0.18 0.01 0.05 0.02 0.02 0.05 0.05 0.02 0.17 0.04 <0.01 0.01 0.04 0.05 0.13 1.84 1.7 2.02 2
Tm 0.03 <0.005 0.01 <0.005 <0.005 <0.005 0.005 <0.005 0.02 <0.005 <0.005 <0.005 0.05 0.01 0.02 0.29 0.26 0.32 (0.33)
Yb 0.18 0.03 0.05 0.02 0.04 0.02 0.03 0.01 0.13 0.02 <0.01 <0.01 0.03 0.05 0.13 1.65 1.65 1.93 2.01
Lu 0.03 0.004 0.01 0.004 0.005 0.004 0.005 0.003 0.02 <0.002 <0.002 <0.002 0.01 0.01 0.02 0.26 0.26 0.31 0.32
(Eu/Eu*)N 1.17 - 1.88 1.83 1.22 9.15 0.88 1.32 6.55 0.35 1.47 1.44 0.26 1.85 2.52
(Ce/Ce*)N 0.75 0.99 0.49 0.48 0.93 0.49 0.66 0.73 0.67 0.76 0.77 1.13 0.71 0.65 1.03
(La/Yb)N 3.89 1.59 14.95 34.99 3.79 58.7 29.11 351.5 108.6 650.5 - - 47.53 6.52 3.34
Rare earth element (REE) concentrations measured in representative rock samples are
listed in Table 2. Chondrite-normalized REE patterns are plotted in Fig. 6. With one
exception, europium anomalies (Eu/Eu*) are higher than in seawater, but still below those of
the vent fluid (Fig. 5). Apart from three samples, most of the mineralised rocks display
relevant positive Eu anomalies (1.22 to 9.15), suggesting interaction with hydrothermal
fluids. Serpentinites, stockwork and steatites present flat HREE normalised values at about
chondrite abundances. Serpentinites show a nearly flat LREE pattern, parallel to seawater
with a weak Ce negative anomaly (Fig. 6a). Stockwork samples depict LREE enrichment
over HREE, somewhat comparable to vent fluid REE patterns, with a slight negative Ce
anomaly (Fig. 6b). REE patterns of semi-massive sulphides clearly reflect the vent fluid REE
pattern with significant LREE enrichment and positive Eu anomalies. Cu-rich massive
sulphides are REE depleted (Fig. 6d). Steatite and rodingitic material show LREE patterns
similar to stockwork, but with no Ce anomaly (Fig. 6c).
19
Fig. 6 Chondrite normalized REE patterns for selected representative Rainbow vent field rocks. a Serpentinites.
b Stockwork. c Steatite and rodingitic material. d semi-massive and massive sulphides. Data in Table 3.
Normalising values after McDonough and Sun (1995) *Rainbow vent fluid data (x10) and seawater data (x100)
after Douville et al. (2002)
20
Mineral Chemistry
438 microprobe analyses were completed, 308 of them in sulphides. Chemical compositions
of pyrite, pyrrhotite, ISS/Ccp and sphalerite from the different rock types are reported in
Table 3.
Table 3 Electron microprobe analyses (average values in wt%) of pyrite, pyrrhotite, ISS/Ccp and sphalerite
from the Rainbow vent field. Abbreviations: N number of microprobe analyses, (-) not detected, SMS semi-
massive sulphides, MSch I-II massive sulphide chimneys type I and II, Py pyrite, ISS/Ccp intermediate solid
solution with exsolved chalcopyrite lamellae, Po Pyrrhotite, Sp sphalerite and Marm marmatite
Mineral Lithology Fe Cu S Ag Zn Cd Ni Co Total N
Pyrite Early Stock work 43.6 - 50.5 - - - 3.46 0.10 97.8 3 Stock work 46.3 - 52.9 - 0.01 - 0.32 0.03 99.5 34 Steatite 46.2 0.01 53.2 0.01 0.01 - 0.02 0.20 99.6 8 SMS 46.4 0.10 53.3 0.01 0.04 0.01 0.03 0.18 100.1 45 MSch I 45.3 - 52.5 - 0.30 - - 0.12 98.26 4
Pyrrhotite Stockwork 59.7 0.01 39.4 0.01 0.03 0.03 0.38 0.04 99.58 19 Steatite 59.8 0.11 38.6 0.01 0.02 0.02 0.10 0.33 99.04 8 SMS 60.9 0.02 38.4 0.02 0.02 0.03 0.11 0.06 99.55 24 MSch II 59.6 0.02 38.8 0.02 0.19 0.02 0.01 0.40 99.09 3
ISS/Ccp Stockwork ISS 40.7 22.3 35.7 0.02 0.66 0.03 0.26 0.09 99.71 8 Ccp 31.0 33.6 35.5 0.03 0.36 0.02 0.13 0.03 100.6 9 Steatite ISS 39.7 23.9 35.0 0.01 0.19 0.02 0.05 0.41 98.39 4 Ccp 32.3 29.9 34.6 0.01 0.21 0.03 0.03 0.22 98.70 3 SMS ISS 40.8 20.4 37.7 0.01 0.31 0.03 0.07 0.07 99.39 11 Ccp 32.3 32.3 35.2 0.01 0.13 0.02 0.04 0.20 100.2 30 Cu-MS ISS 39.2 24.7 35.4 0.01 0.03 0.02 0.01 0.52 100.0 13
MSch I Ccp 30.8 33.1 34.8 0.02 1.21 0.02 0.01 0.02 99.98 5 MSch II ISS 42.2 21.7 36.0 0.03 0.16 0.01 - 0.34 100.4 8 Ccp 34.5 29.9 35.3 0.07 0.24 0.06 0.02 0.18 100.2 1
Sphalerite Stockwork Marm. 13.4 0.04 33.7 0.01 52.9 0.11 0.01 0.01 100.2 20 Sp. 3.15 0.05 33.0 0.00 63.2 0.12 0.01 0.00 99.60 10 MS-ch type I Marm. 10.7 0.90 32.6 0.07 53.5 0.33 - 0.05 98.16 5 Sp. 1.40 0.22 31.8 0.10 62.8 0.06 - 0.01 96.45 5 MS-ch type II Marm. 15.6 0.36 34.9 0.01 49.7 0.09 - 0.27 99.95 3
Fe/S ratios in pyrite are fairly constant except in the early stockwork where pyrite is
Ni-rich, containing up to 3.46 wt% Ni and 0.1 wt% Co. Fig. 7 illustrates the Co and Ni
variation in pyrite minerals from the Rainbow vent field. Stockwork contains Ni-rich pyrite
whose Ni contents decrease gradually towards the semi-massive sulphides and massive
sulphide chimneys. In contrast, low Co concentrations found in stockwork pyrite increase
towards the semi-massive sulphides and massive sulphide chimneys. Hence, pyrite from
stockwork has Co/Ni < 1 and the remaining pyrites from the semi-massive sulphides, massive
21
sulphide chimneys and steatites are Co-rich with Co/Ni >1. Similar Co/Ni ratio variation is
observed for pyrrhotite, ISS/Ccp and sphalerite with the exception that pyrrhotite from semi-
massive sulphides still reveals Co/Ni <1.
Fig. 7 Co versus Ni (at %) plot for pyrite from the Rainbow vent field. Pyrite from MS-ch I is not represented
since Ni is absent. The black star represents the average value of pyrite from the serpentinite-hosted Eastern
Metals deposit (Auclair et al. 1993). The compositional field where Co/Ni >1 (shaded area) delimits pyrites of
volcanogenic association (VMS) (Loftus-Hills and Solomon 1967; Bralia et al. 1979; Mookherkee and Philip
1979; Walshe and Solomon 1981; Bajwah et al. 1987; Lawrie and Miller 2000)
ISS/Ccp chemistry is complex, although comparable to that of the Bent Hill and ODP
mounds on the Juan de Fuca Ridge (Lawrie and Miller 2000). Fe and Cu proportions vary
widely (Fe ~42.16 wt% to ~30.75 wt%, Table 4) due to chalcopyrite exsolution lamellae.
These non-stoichiometric concentrations suggest that no equilibrium has been attained in the
newly formed sulphides. Zn contents in the chalcopyrite portions are variable, ~0.36 wt% in
St, ~0.13 wt% in SMS, ~1.21 wt% MS-ch I, ~0.24 wt% MS-ch II and 0.21 wt% Stea. The
same defines the ISS portion which contains ~0.66 wt% in St, ~0.31 wt% in SMS, 0.03 wt%
in Cu-MS, 0.16 wt% in MS-ch II and 0.19 wt% in Stea. Experimental work by Kojima and
Sugaki (1985) simulating hydrothermal conditions for the Cu-Fe-Zn-S system between 300ºC
22
and 500ºC and pressures of 500kg/cm2, confirmed that the Zn content of ISS and chalcopyrite
decreases with decreasing temperature, as previously observed by Hutchinson and Scott
(1981). If we consider either of the main mineralising sequences, St – SMS – MS-ch II and St
– Stea, Zn contents in ISS/Ccp decrease upwards, which suggests a decrease in temperature,
as is to be expected. This does not apply to Cu-MS and MS-ch I units. The first, which is Zn-
free, probably results from recrystallization and zone refining inducing Zn remobilisation,
whereas Ccp in MS-ch I shows anomalously high Zn contents, possibly due to small
sphalerite inclusions. Because ISS can easily turn into chalcopyrite, the regular presence of
ISS in Rainbow indicates rapid cooling with effective sealing (Kase et al. 1990). There are
two distinct populations of sphalerite according to FeS contents. Lower FeS values (2.9 to 3.4
wt% in St and 2.45 to 0.9 wt% in MS-ch I) correspond to sphaleritess associated with pyrite.
High FeS values (10.41 to 17.9 wt% in St to 14.41 to 17.29 wt% in MS-ch II) correspond to
sphalerite (marmatite), related to pyrrhotite – ISS/Ccp assemblages. Low FeS in sphalerite
suggests increasing sulphur activity (aS2), decreasing temperature, or the combination of both
(Vaughan and Craig 1997). Marmatite (Cu-rich) occurs in MS-ch I associated with ISS/Ccp
(Zn-rich), suggesting the presence of minute inclusions of ISS/Ccp within marmatite and
interference during analytical processes. Sphalerite specimens from sulphide chimneys
contain relevant amounts of Cu, Ag and Cd (MS-ch I) or Cu and Co (MS-ch type II).
Variation in the FeS contents from sphalerite, if in equilibrium with pyrite or pyrrhotite,
suggest that their precipitation occurred under different conditions, most probably at different
temperatures. This may reflect different degrees of mixing of hotter vent fluid with cold
seawater on proximal and distal areas of the hydrothermal system.
23
Nd isotopes
Radiogenic isotopes are powerful tools for identifying sources and processes, and will
be used in conjunction with trace element data to identify the fluid source(s). The Nd isotope
compositions of 12 samples from the Rainbow sulphide deposit are reported in Table 4 and
plotted against Co/Ni and 1/Nd in Figs. 8 and 9. The high Nd (and REE) concentrations of
the semi-massive sulphides compared with the other mineralised samples are, most probably,
related to the occurrence of phyllosilicates since REE are not easily incorporated in sulphides.
This would also explain the REE-depletion in silicate-free massive sulphides (Table 2).
143Nd/
144Nd ratios of all samples, apart from serpentinite, vary in a narrow range
between 0.513025 and 0.513082, corresponding to a εNd of +7.55 to +8.66. Unmineralised
serpentinite, however, presents a very distinctive Nd isotopic composition at 0.512926 (εNd =
+5.62). This serpentinite exhibits the lowest εNd and Co/Ni ratio for 1/Nd equal to 1.
Stockwork samples exhibit low Co/Ni ratios (Co/Ni<1) and increasing εNd values (+7.55 -
+8.52) and 1/Nd ratios (1.91 to 8) while the semi-massive sulphides show very uniform εNd
values between +8.15 and +8.52 and high Co/Ni ratios (Co/Ni>5) and variable 1/Nd ratios
(0.22 - 1.75) (Fig. 8). Rodingitic material and part of the stockwork exhibit lower εNd and
Co/Ni values when compared to the semi-massive sulphides.
24
Table 4 Nd isotope data from representative samples of the Rainbow vent field
Sample Rock type Nd (ppm) 1/Nd
143Nd/144Nd ±2sE εNd
IR-DR-02-L-01 Rodingitic Material 0.47 2.13 0.513030 16 7.65
IR-DR-03-E Steatite 0.11 9.09 0.513066 53 8.35
SH1-DR3-8-3 Steatite 0.45 2.21 0.513082 16 8.66
IR-DR-01-G-02 Semi-massive sulphide 0.57 1.75 0.513056 8 8.15
IR-DR-01-C-01 Semi-massive sulphide 4.52 0.22 0.513069 8 8.41
SH1-DR3-2-1 Semi-massive sulphide 3.54 0.28 0.513075 6 8.52
SH1-DR3-2-3 Semi-massive sulphide 1.78 0.56 0.513069 9 8.41
SH1-DR3-9-9 Stockwork - - 0.513053 15 8.10
SH1-DR3-9-4 Stockwork 0.52 1.91 0.513073 6 8.49
IR-DR-02-C Stockwork 0.13 8.00 0.513075 55 8.52
IR-DR-03-F Stockwork 0.30 3.33 0.513025 26 7.55
IR-DR-01-A-05 Serpentinite 0.93 1.08 0.512926 6 5.62
Fig. 8 εNd versus bulk-rock Co/Ni ratios in selected samples from the Rainbow vent field
26
Discussion
Serpentinisation and heat generation
Serpentine textures and mineralogy reveal that serpentinisation has taken place sub-
seafloor under low-temperature retrograde metamorphism. Olivine reacted first, and more
extensively than pyroxene, producing pseudomorphic (lizardite-dominated) textures.
Considering the latter observations, temperatures were certainly below 350ºC (Moody 1976;
Janecky and Seyfried 1986) and probably under 270ºC (O’Hanley 1996; Allen and Seyfried
2004). Serpentinisation reactions are exothermic (MacDonald and Fyfe 1985; Fyfe 1992;
O’Hanley 1992; Kelley et al. 2001; Lowell and Rona 2002; Schroeder et al. 2002; Allen and
Seyfried 2004) but are unlikely to produce hydrothermal temperatures above a few tens of
degrees Celsius. In order to produce hydrothermal fluids slightly above 100ºC,
serpentinisation rates need to be extremely high (100 Kg/s) and flow rates low (10 Kg/s)
(Lowell and Rona 2002). At the Rainbow the opposite occurs: flow rates are high (500Kg/s)
and serpentinisation rates are probably low (~ 0.1 Kg/s) except when, episodically, new
reaction surfaces are available (MacDonald and Fyfe 1985; Thurnherr and Richards 2001;
Lowell and Rona 2002). Thus, it is extremely unlikely that serpentinisation reactions alone
can produce the reported high temperatures at the Rainbow vents. Furthermore, the
hydrothermal sulphide mineralisation event indisputably overprints the serpentinisation event
thus any considerations regarding the vent fluid properties cannot be directly attributed to
serpentinisation reactions alone. Consequently, the presence of a magmatic heat source at
depth as previously proposed by several authors (Cave et al. 2002; Lowell and Rona 2002;
Mével et al. 2003; Allen and Seyfried 2004) prevails as a reasonable explanation.
Sulphide mineralisation
27
Serpentinites are naturally Cu and Zn depleted but the textures and chemistry of the
sulphide-bearing rocks found at Rainbow show conspicuous similarities with mafic VMS
deposits. Accordingly, the Rainbow massive sulphide ore-grade deposit is Cu and Zn-rich (28
wt% Cu; 5 wt% Zn), but Pb depleted whereas the upper parts of the hydrothermal system are
also extremely Co-enriched (up to ~6300 ppm) with high Co/Ni ratios. These elements are
characteristic of mafic-hosted seafloor hydrothermal systems. It is reasonable to assume that
a mafic metal source is responsible for Cu, Zn and Co input, along with Fe, into the system.
Cu-Fe sulphide chemistry data suggest that temperature decreases from deeper stockwork
through sub-surface semi-massive sulphides to massive-sulphide chimneys.
Ni and Co are remobilised, during serpentinisation, from primary minerals into
serpentinite-group minerals and later, during sulphide mineralisation, into sulphides
(Marques et al. 2003). While Ni gradually decreases, Co increases sharply with the evolution
of the hydrothermal system (Srp-St-Stea and Srp-St-SMS-MS) indicating Co input. These
observations apply both to bulk-rock and sulphide chemistry data. Co/Ni ratios have been
used as indicators discriminating sulphide deposits of different settings. At Rainbow, Co/Ni
ratios found in stockwork pyrite compare well to those of pyrites from the serpentinite-hosted
Eastern Metals deposit (Auclair et al. 1993) whereas the Co-rich pyrite with Co/Ni ratios >1,
found in semi-massive and massive sulphides, match those of volcanogenic pyrite found in
VMS deposits from different localities (Loftus-Hills and Solomon 1967; Bralia et al. 1979;
Mookherkee and Philip 1979; Walshe and Solomon 1981; Bajwah et al. 1987). Similar Co/Ni
ratio variation has been pointed out in the Outokumpu deposit ores, with Co/Ni ratios typical
of VMS deposits except for a few Ni-rich pyrites found in the deeper parts of the deposit,
which have been interpreted as earlier sulphides (Loukola-Ruskeeniemi 1999).
In the Rainbow vent field, Co/Ni ratios seem to be excellent tools for discriminating
between the various units.
28
LREE enrichment occurs alongside sulphide mineralisation reaching a maximum in
semi-massive sulphides and decreasing again in the massive sulphide units. The presence of
altered serpentinite relics in semi-massive sulphides may favour the observed LREE
retention. The REE-depleted character of the unmineralised serpentinites and the increasingly
LREE enrichment in the sulphide bearing rocks strongly suggest that the source of metals
(Cu, Zn and Fe) transported by the hydrothermal fluid, should also contribute with LREE.
HREE patterns are nearly constant throughout the system compatible with the higher LREE
mobility as Cl complexes in acidic solutions (Mills and Elderfield 1995; Douville et al.
1999).
Nd isotopes
Hydrothermal fluids that were originally seawater have been progressively modified
via (direct or indirect) magmatic interactions during percolation. The sulphides formed during
the hydrothermal sulphide mineralisation episode acquired the Nd isotopic signature of the
hydrothermal fluid from which they derived. From the above sections, there is clear evidence
that the sulphide-bearing serpentinites (stockwork and semi-massive sulphides) and related
rocks (steatites and rodingitic material) result from the same peridotite precursor as the
serpentinite. However, it is the combination of the serpentinisation process with the presence
of a magmatic heat source at depth (e.g. intrusion of a gabbroic body) that may explain the
chemical characteristics of the Rainbow site (Cave et al. 2002; Lowell and Rona 2002; Mével
et al. 2003; Allen and Seyfried 2004). The contribution of two sources (seawater and
magmatic) in the hydrothermal fluid at Rainbow is also supported by Nd isotopic
compositions and Co/Ni and 1/Nd ratios (Fig. 8, 9).
At the Rainbow vent field, mineralised samples are characterised by εNd values of
+7.55 to +8.66, but no Nd isotope ratios have, so far, been measured on Rainbow
hydrothermal fluids. Basalts in the vicinity of the Rainbow vent site (South AMAR segment
29
between 36.29ºN and 35.67ºN) exhibit εNd values at +8.56 to +9.95 (143
Nd/144
Nd = 0.513077 -
0.513148) which correspond to the top end of our sample variation range (Fig. 8, 9; Dosso et
al. 1999). In contrast, the present day deep-water circulation in the Rainbow rift valley is
characterized by an inflow of North East Atlantic Deep Water, from the eastern ridge flank,
with εNd value of -12.4 (Thurnherr and Richards 2001; Thurnherr et al. 2002; Lacan 2002). In
comparison, the East Pacific Rise (EPR) hydrothermal fluids present εNd values of +5 to +7
(Michard and Albarede 1986) lower by 2 εNd values than those of the oceanic crust at that
location. Mineralised rocks and oceanic crust from the Rainbow area exhibit similar εNd
variations to the EPR, contrasting with the Nd isotope signature of the seawater within the
area.
The low εNd value of the unmineralised serpentinite probably results from simple
peridotite-seawater interaction as supported by the low Co/Ni ratio (low Co/Ni ratio in
peridotite and seawater, see Niu et al. 2001). Subsequently, a higher temperature
hydrothermal fluid with high Co/Ni ratios and radiogenic Nd isotopic composition is
gradually superimposed on the previous regime. This leads to the formation of sulphide
mineralisation during focused fluid upflow, mainly under the seafloor. This would explain the
mixing trend in the stockwork samples and rodingitic material that preserve characteristics of
the earlier serpentinisation-related fluid regime (low Co/Ni, unradiogenic Nd isotope), mixed
with the later mineralising fluid regime (high Co/Ni, radiogenic Nd isotopic composition).
The non-mixing trend in the semi-massive sulphides is generated by the high εNd and Co/Ni
values and low 1/Nd. The emplacement of a gabbroic body (with N-MORB affinity) within
the serpentinites can provide heat and elements which easily explain the isotope signatures
and all other peculiarities found in the Rainbow vent field.
Vent fluid chemistry
30
Rainbow fluids hold the highest reported chloride concentration, the lowest end-
member pH, high trace metal contents and high REE, K, Rb and Cs contents (Douville et al.
2002). Experiments on simple peridotite-seawater interactions at temperatures below 350ºC
produced high pH fluids (neutral to alkaline), rich in H2, CH4, H2S, Mn and SiO2 with low
dissolved metal contents (Seyfried and Dibble 1980, Berndt et al. 1996; Donval et al. 1997;
Charlou et al. 1997; Kelley et al. 2001; Palandri and Reed 2004). At temperatures of ~400ºC,
fluids are initially acidic (pH increasing afterwards) with high dissolved Fe, Ca and SiO2
(Seyfried and Ding 1995; Allen and Seyfried 2003, 2004, 2005). Furthermore, Wetzel and
Shock (2000) have calculated that simple peridotite-seawater interaction would produce a
fluid with less than 50% K and ~95% SiO2 than the fluid resulting from basalt-seawater
reactions. These experiments could not reproduce the chemistry of the Rainbow vent fluids,
most likely because they were based on the assumption that serpentinisation occurs at high
temperatures, with olivine stable over pyroxene and antigorite as the stable serpentine phase.
On the other hand, experiments on basalt-seawater interactions, at ~400ºC, produced a
low pH fluid, rich in metals (Fe > Mn > Zn > Cu.), H2S, LREE (La, Ce and Eu) and CaO
(Seyfried et al. 1986; Bischoff and Rosenbauer 1989; Bienvenu et al. 1990; Seewald and
Seyfried 1990; Palandri and Reed 2004). These results are comparable to the composition of
vent fluids from modern basalt-hosted high temperature systems and, remarkably, to the
Rainbow vent field (Douville et al. 2002; James et al. 1995; Seewald and Seyfried 1990).
High mFe/mCu and mFe/mMn coupled with high Fe contents in the Rainbow vent
fluids are indicative of reducing, acidic and high temperature conditions at the sub-seafloor
reaction zone. However Zn content in fluids at temperatures >200ºC do not seem to be
temperature dependent (Seewald and Seyfried 1990; Large 1992; Hannington et al. 1995;
Seyfried and Ding 1995; Charlou et al. 2000; Douville et al. 2002; Allen and Seyfried 2003).
Thus the profusion of Zn in the Rainbow fluids is most probably related to source rock
composition and/or brine/seawater mixing since there is no evidence for sphalerite
31
dissolution (Metz and Trefry 2000; Douville et al. 2002). The presence of Co is related to
high temperature Cu-rich VMS deposits or even lower temperature fluids, if chlorinity is high
(e.g. Besshi-type VMS; Peter and Scott 1997).
The Rainbow end-member fluid is chlorine-rich (750mM Cl) - surpassing seawater
chlorinity (546mM Cl) (Douville et al. 2002). The critical point for seawater is 407ºC and
298.5 bars (Bischoff and Rosenbauer 1988) indicating that the Rainbow fluids, venting at
365ºC and 2400 m depth, have crossed the two-phase boundary below the critical point at
minimum temperatures of 380ºC (Charlou et al. 2000; Douville et al. 2002). Under sub-
critical conditions, boiling seems to be the predominant phase separation process causing the
exsolution of volatile components (CO2, H2S, CH4, SO2, H2) into the vapour phase, with
drastic changes in the pH and fO2 of the system (Drummond and Ohmoto 1985). Sub-critical
boiling results in only moderate increase in salinity (Charlou et al. 2000), which is possibly
not enough to explain the high-chlorinity values at Rainbow. Under these conditions, mixing
with a high salinity brine or the direct magmatic contribution of a Cl-rich metal-bearing
aqueous fluid would explain the high salinities and enriched metal contents.
32
Conclusions
Textural evidence of sub-seafloor hydrothermal replacement at the Rainbow vent field
are indicative of a highly efficient process of trapping metals, essential for the formation of
an ore–grade deposit under the seafloor. The new geochemical data, combined with the
current knowledge at Rainbow, presents compelling evidence for the presence of a gabbroic
intrusion at depth, contributing heat, metals and other elements. The evolution of the system
may be summarized as follows:
(1) A hot gabbroic body has intruded (near) a completely serpentinised peridotite
(2) The resulting reaction zone is characterized by high temperature, acidic and reducing
conditions favouring metal leaching and efficient metal transport by Cl-rich hydrothermal
fluids.
(3) The gabbroic body contributes heat and metals to sustain the hydrothermal circulation and
provide the mafic VMS character of the sulphide deposit.
(4) Strongly positive CH4 anomalies represent contributions from secondary peridotite-
seawater reactions in the surrounding areas.
33
Acknowledgments
The authors wish to thank Claudio Cermignani (University of Toronto) for technical and
analytical support with the EPMA, and Prof. Martin Sinha (NOC), Gabriela Henriques and
Yannick Beaudoin for improving the English. The manuscript benefited significantly from
constructive comments and suggestions from Bernd Lehmann. Fundação Para a Ciência e
Tecnologia (FCT) provided funds for the study through Project SEAHMA (FFCUL/FCT-
PDCTM/ P/MAR/ 15281/ 1999) and a Ph.D. scholarship (SFRH/BD/2978/2000) given to
A.F.A. Marques. A.F.A. Marques also benefited from a student research grant from the
Society of Economic Geologists Foundation. V. Chavagnac was funded by the 2000 National
Oceanography Centre, Southampton Research Fellowship. The NERC core strategic science
funds the hydrothermal research at NOCS.
34
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